drm/i915: Reject privileged commands

The spec defines most of these commands as privileged. A few others,
like the semaphore mbox command and some display commands, are also
reserved for the driver's use. Subsequent patches relax some of
these restrictions.

v2: Rebased

Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>